The present invention relates to a new and improved construction of circuit for speed or velocity measurement of vehicles according to the Doppler-radar principle, wherein a Doppler-useful signal is evaluated in a computer to which it is delivered by means of a threshold value switch.
In order to influence the vehicle speed during railroad-freight car-shunting operations there are employed track brakes. To control these track brakes the velocity of the railroad cars must be continuously measured with high accuracy. In this respect it is known, as taught for instance in the publication "Eisenbahningenieur" 17 (1966) 5, in the article entitled "Geschwindigkeitsmessungen mit Radar in automatischen Ablaufanlagen", to use for this purpose radar devices which work in accordance with the Doppler principle.
During velocity measurements carried out with such heretofore known radar installations, electromagnetic waves are produced in a small transmitter and transmitted by means of a radar antenna. A vehicle within the measuring region reflects part of the impinging energy. The reflected signal is received by the antenna and mixed in a mixer with the transmitted signal. At a subsequently connected low-pass filter and an amplifier there is filtered out of the mixed product the Doppler frequency and measured. Consequently, the magnitude of the vehicle velocity in m/s can be calculated from the equation f.sub.d .apprxeq.6.67.multidot.f.multidot.v.multidot.cos .alpha. wherein f.sub.d is the Doppler frequency in Hz, .alpha. is the angle between the transmitted beam and the direction of movement of the vehicle, and f is the transmitter frequency in GHz.
In this known radar installation the Doppler frequency signals are transformed by means of a dual or double range-threshold value switch into squarewave or rectangular pulses which are further processed in an evaluation circuit or in a computer.
Such prior art velocity measuring device has been found to be unfavorable as concerns the sensitivity of the device, especially for railway-freight car-shunting operations, since the fixed predetermined response thresholds must be selected to be considerably wide apart in order to avoid the effects of noise signals. Additionally, such equipment cannot bring about any gap fill during fading and cannot smooth phase shifts, such as for instance occur during slight pivotal or rocking movements of the freight cars.
It is also known to the art from German patent publication No. 2,324,271 to incorporate into a circuit for the measurement of the displacement path or velocity of vehicles, while utilizing a Doppler-radar device, a synchronizable oscillator after the threshold value switch and directly in front of the evaluation circuit. This oscillator must be capable of being synchronized over a wide frequency range.
With such circuit it is possible to correct phase shifts or changes and amplitude breakdown. But this circuit is still disadvantageous as concerns sensitivity. If the noise is eliminated by having widely spaced response thresholds of the threshold value switch, then the circuit becomes extremely insensitive, whereby frequently it is not possible to obtain any synchronization signal, during a longer time span, for the synchronizable oscillator. On the other hand, if the response thresholds are selected to be just slightly above the noise level, then the effect of noise in the synchronizable oscillator must be eliminated by a low-pass filter. As a result, the circuit becomes too slow or sluggish, since the time-constant of the low-pass filter must be relatively large in order to be able to also process extremely small Doppler frequencies up to several Hz.